1. Field of the Invention
The present invention relates to a method of determining a relative permittivity of concrete, in particular in dependence on its moisture content and according to which a radar wave is emitted into a concrete mass, and a reflected radar signal, which is generated in the concrete mass and, in particular, from foreign bodies contained in the concrete mass, is electronically processed and is subjected to an evaluation. The present invention also relates to use of the method of determining of the permittivity for depth scaling of the radar data, in particular, for determining presence of foreign bodies in the used constructional material with an electromagnetic sensor.
2. Description of the Prior Act
In the field of non-destructive material testing in the microwave frequency region, different methods of determining of the permittivity of an inspected medium are known. In this field, the inspection of concrete dependent on its composition and/or moisture content is of a particular interest.
In the article of Bungey et al. "The influence of concrete composition upon radar test results," "Non-Destructive Testing in Civil Engineering," The British Institute of Non-Destructive Testing, INSINT, vol. 39, No. 7, 1997, pp. 4874-478 (Bungey) among others, determination of dielectric properties of concrete by laboratory measurements is described. To this end, preliminary prepared concrete samples are placed inside of a large volume microwave conduit oncluded at one end with a predetermined load resistance, and their permittivities or dielectric constants and conductivities arc determined.
In the article of Padartz, et al., "Coupling Effects of Radar Antennae on Concrete", Non-Destructive Testing in Civil Engineering, The British Institute of Non-Destructive Testing, NDT-CE '97, vol. 1, pp. 237-245 (Padartz), placing of plate-shaped concrete samples between a radar transmitter and a radar receiver (please see FIG. 1a) and the determination of the propagation or the frequency-dependent shifting of a spectral density of a receiving signal are described.
Determination of an appropriate moisture content on an inspected bottom surface with GPR (Ground Penetrating Radar) during geodesic works is described in an article of Berktold et al., "Subsurface Moisture Determination With the Ground Wave of GPR", R. G. Plumb GPR '98, University of Kansas (Berktold). Here, the difference in propagation of reflections of an emitted radar wave from two known reflectors located at different depths (please see FIG. 1b) is evaluated with an algorithm.
Finally, a method of an iterative migration according to which a determined value of a dielectric constant or permittivity .epsilon..sub.r is obtained with a focused migrated profile of recorded radar data is described in an article of Fisher et al., "Examples of Reverse-Time Migration of Single-Channel, Ground-penetrating Radar Profiles", Geophysics, vol. 57, No. 4, 1992, pp. 577-586.
However, the use of the known methods for the determination of the moisture content by determining a relative permittivity in practice, e.g., on a constructional site, is connected with certain difficulties. This is because additional measurements are required, which is associated with increased costs, or as, e.g., with the method described in Berktold, the knowledge of the structure or the texture or the surface layer is required. With the method of Padaratz, a two-side access to the inspected samples for the transmission measurement is needed.
Accordingly, an object of the present invention is to provide a method of determining the constitution and/or the moisture content of concrete by determining the relative permittivity of the concrete which would not require any preliminary knowledge about the inspected material, provision of samples and/or additional separate measurements.